This invention relates generally to axle transducers for weighing the contents of cargo carrying vehicles such as grain wagons. The invention deals more particularly with a weigh axle transducer which is constructed to take advantage of shear beam techniques.
As disclosed in U.S. Pat. No. 3,650,340 to Bradley, grain carts and other wheel mounted containers that are used for various agricultural applications are sometimes equipped with an electro-mechanical system for weighing the contents of the container. In the arrangement shown in the Bradley patent, strain gauges are mounted on an axle which functions as a beam that deflects in proportion to the loading of the container. The beam deflection is sensed by the strain gauges and used to generate an electric output signal which is proportional to the deflection and thus to the weight of the container.
Although bending beam transducers of this type function in a generally satisfactory manner in low capacity applications, they are not well suited for use in the weighing of heavy loads. Because the strain gauges must be spaced along the beam length, the beam must have adequate length between the container and wheel to accommodate the gauge spacing that is necessary to obtain a sufficient electrical signal from the gauges. With a large capacity container which is subject to heavy loads, structural considerations dictate that the beam have a sizeable diameter in order to provide a moment of inertia adequate to compensate for the considerable bending moment that results from the heavy loading. However, the large beam diameter reduces the deflection of the beam, and the strain gauges must be spaced farther apart in order to provide an acceptable signal. This in turn requires a longer beam which increases the bending moment and necessitates that the beam diameter be increased in order to compensate for the increased bending moment. Consequently, it is apparent that the structural need for increased beam diameter in large capacity applications works directly against the ability of the beam transducer to generate an adequate signal. Thus, in large capacity applications, the beam at best must be unduly large at least, and this makes it expensive, difficult to manufacture, and undesirable in other aspects. At worst, it is impossible in a heavy capacity machine for the beam to meet the requisite design criteria of generating an acceptable signal and providing adequate structural strength at the same time.
In various industrial application, shear beam transducers have been used on stationary weighing equipment. As disclosed in U.S. Pat. No. 4,459,863 to Nordstrom, I beams are particularly advantageous because the shear strain in the web area of the beam is high and relatively uniform, while the bending related stresses are relatively low in the web. Nevertheless, I-shaped shear beams have not to my knowledge been used in the past in weigh axle transducers or any other mobile scale application. No doubt, one reason is that the I beam configuration is ill suited for the cylindrical beam shape that is characteristic of axles and spindles on which wheels are mounted. The I beam configuration can be formed on a cylindrical spindle by forming a pair of blind openings in its opposite sides. However, if the openings are made large enough in diameter to provide the web of the I beam with sufficient shear strain to generate an acceptable signal level, so much material must be removed from the spindle that its moment of inertia about the web axis is inadequate for the anticipated axle loading. Conversely, if the openings are made small enough to maintain a suitable structural factor of safety, then the shear strain in the web is too low to provide an adequate signal.